The present invention relates generally to a scanning particle microscope and, more specifically, to an electrode arrangement in the particle beam source.
Scanning electron microscopes are being employed to an increasing degree in the semiconductor industry in the manufacture and development of micro-electronic components. For example, the individual process steps involved in the manufacture of large scale integrated (LSI) circuits can be monitored, masks and wafers can be inspected, or microstructures can be generated on a subject by electron beam lithography. Scanning electron microscopes are also used for checking electrical properties of large scale integrated circuits. By measuring the time variations in the voltage at selected circuit nodes, errors in the structure of the components under investigation can be recognized and eliminated early in the design phase.
To avoid errors during voltage measurements with scanning electron microscopes, care must be taken not to charge the surfaces of the components, which are usually disposed on an insulated carrier substrate for the duration of the measurement. Therefore, during mensurational applications, the primary electron current incident on a component must be substantially equal to the current of the secondary and backscatter electrons being emitted from it.
Such condition is met when the primary energy of the electrons coincides with a neutral energy point. The neutral energy point is material-dependent and is relatively low, typically within the range of 0.5-2.5 keV. Use of such low-energy electrons in measurement applications offers the further advantage of load-free and damage-free testing of radiation-sensitive components, such as, MOS memory units.
In electron beam lithography where microstructures are currently being generated with high energy electrons, the use of significantly lower primary electron energies will have to be utilized to improve resolution.
Thus, there is an increasing need for efficient low-voltage scanning electron microscopes in all areas of semiconductor technology for rapid and high-resolution investigation of LSI components. Conventional scanning electron microscopes having low acceleration voltages can only be used to a limited extent for such purposes, since the available resolution is poor and the probe current on the specimen surface is considerably diminished.
The reason for such characteristics is a result of electron-electron interaction which opposes the focusing of the electron beam. E1ectron beam expansion is a consequence of the Boersch effect which is a particular problem in electron sources that generate highly directional beams such as, for example, lanthanum hexaboride single crystal cathodes, as a result of which fine electron probes with high current densities cannot be generated solely with low accelerating voltages.
The smallest achievable probe diameters on a specimen is limited by two fundamentally different effects. Firstly, lateral Boersch effect is a result of the Coulomb repulsion between electrons along the entire beam path which increases their mean spatial distance, and consequently, the beam diameter. Secondly, the energetic Boersch effect results from the electromagnetic interaction between electrons in regions of high current densities and, in particular, at beam crossing points. This second effect leads to a spread in energy distribution so that the probe diameter is indirectly enlarged by chromatic aberration of the objective lens.
Reference is made to pending U.S. Patent application Ser. No. 751,020, filed July 2, 1985, and based on German Patent application P 34 29 804.5, which proposes a scanning electron microscope with a diminished lateral Boersch effect. The lateral Boersch effect is diminished by first enabling electrons to traverse the electro-optical column of the scanning microscope at high potential and then, shortly before the electrons reach the specimen, decelerating the beam to a desired low energy. As a consequence of the high accelerating voltage, however, the width of the energy distribution of the electrons increases, particularly at the source beam crossing point, so that the reduced influence of the lateral Boersch effect on the probe diameter is at least partially cancelled by the increased energetic Boersch effect.